With the advent of portable computing there has arisen of need for a motion sensors capable of detecting extreme or sudden accelerations. By way of example, rigid disk drive units housed within a portable computer are extremely sensitive to certain types of displacements. If data is being written to the disk surface by a transducer, and the drive or computer is subject to physical shock (as maybe the case if the device was suddenly dropped) the resulting motion could case the transducer to be knocked off the data track that is currently being written. Obviously, off-track writing would produce highly undesirable data errors.
In a disk drive unit, an actuator arm is employed to position one or more transducers over a concentric data track of the magnetic medium. Because of their configurations, these actuator assemblies are generally immune to sudden linear accelerations or shock displacements. They are, however, very sensitive to rotational displacements. For example, a rotational acceleration component in the plane of the magnetic disk about the actuator center line essentially causes the disk to be rotated out from the underneath the transducer. If the acceleration is high enough and the drive is writing data to the disk there is a chance that data can be written over adjacent tracks.
In the prior art, a variety of sensors for detecting angular motion have been devised. For example, U.S. Pat. Nos. 4,718,276; 5,067,351; 5,176,030; and 5,170,104 generally describe various types of sensors and control systems for sensing sudden displacements. These sensors, however, are highly complex and, in some cases, require the use of dangerously toxic materials such as mercury.
The prior art also describes numerous other sensor designs. For example, U.S. Pat. Nos. 5,025,336; 4,862,298; 4,933,785; 5,189,575; 4,535,374; 3,984,873; 5,027,241; 3,646,536; 4,686,592; 4,315,289; 4,280,156; 3,786,457; 3,702,997; PCT International Application No. PCT/US91/09367, and IBM Technical Disclosure Bulletin Vol. 25, No. 4, September 1982, show a variety of systems which include sensors sensitive to both linear and rotational accelerations. What is needed, however, is a shock sensor that provides sensing of rotational displacements, while being immune or largely insensitive to linear accelerations in an X or Y direction. Additionally, the sensor should be relatively simple and inexpensive to manufacture and avoid the use of toxic substances.